Plasmolysis is the contraction of cells within plants due to the loss of water through osmosis. It is the cell membrane peeling off of the cell wall and the vacuole collapsing. Plasmolysis occurs when a plant cell's membrane shrinks away from its cell wall. This phenomenon occurs when water is drawn out of the cell and into the extracellular (outside cell) fluid. The movement of water occurs across the membrane moving from an area of high water concentration to an area of lower water concentration outside the cell. It is unlikely to occur in nature, except in severe conditions. Plasmolysis is induced in the laboratory by immersing a plant cell in a strongly saline(salty) or sugary solution, so that water is lost by osmosis. If onion epidermal (outer) tissue is immersed in a solution of calcium nitrate, cells rapidly lose water by osmosis and the protoplasm of the cells shrinks. This occurs because the calcium and nitrate ions freely permeate the cell wall and encounter the selectively permeable plasma membrane. The large vacuole in the center of the cell originally contains a dilute solution with much lower osmotic pressure than that of the calcium nitrate solution on the other side of the membrane. The vacuole thus loses water and becomes smaller. The space between the cell membrane and the cell wall enlarges and the plasma membrane and the protoplasm within it contract to the center of the cell. Strands of cytoplasm extend to the cell wall because of plasma membrane-cell wall attachment points. If plasmolysed cells are not transferred quickly from the salt or sugar solution to water so their vacuole may be refilled, cytorrhysis will begin.